Modulating fuel control system



Sept. 3, 1968 E. w. WILSON E AL 9 MODULATING FUEL CONTROL SYSTEM Filed Oct. 12, 1966 2 Sheets-Sheet 1 F1G.l.

lo f 2o FUEL 16 --SUPPLY- SOURCE 15 BURNERS 14 INVENTORS EDWARD w. WILSON DONALD ,L. WOODEN ATTORNEY Sept. 3, 1968 E. w. WILSON ETAL MODULATING FUEL CONTROL SYSTEM 2 Sheets-Sheet 2 Filed Oct. 12, 1966 8 2 WY V w 3 4. M 3 +2 0 2 C v Ill H 0% a D 3 7 l5 2 i 2 m 9 G W 1 1 o H F 2 T To POWER SUPPLY DONALD L. WOODEN BY I v My Q ORNEY United States Patent 3,399,832 MODULATING FUEL CONTRQL SYSTEM Edward W. Wilson, Philadelphia, and Donald L. Wooden,

Wayne, Pa., assignors to Peerless Industries, Incorporated, Boyertown, Pa., a corporation of Pennsylvania Filed Oct. 12, 1966, Ser. No. 586,254 Claims. (Cl. 236-91) This invention relates broadly to control systems for boiler water heating systems, and the like, and more particularly to a boiler fuel supply control system in which boiler water temperature and boiler input fuel volume are modulated proportionately with variations in outside temperature.

In the heating of buildings and the like with boilerwater heating systems, it has been found that maximum room temperature comfort is obtained when the boilerwater temperature varies inversely to the change in temperature outside the building, that is, as the outside temperature increases the boiler water temperature correspondingly decreases, and as'the outside temperature decreases, the boiler water temperature correspondingly increases to increase the inside room temperature. In such a system the boiler water temperature must be modulated proportionately to the changes in outside temperature to obtain the desired comfortable heating effects. However, to proportionately modulate the boiler water temperature effectively, it has been found that the intensity of the input heat to the boiler must be modulated proportionately to the changes in outside temperature. This can best and most effectively be accomplished by modulating proportionately the volume of boiler input fuel, such as gas or the like, with the changes in outside temperature. While a gas burner will be referred to throughout this specification as the boiler heating means, for purposes of illustration, it is to be understood that the system of the invention is also applicable for use with other types of burners and fuels. The system of the invention thus provides means for varying or modulating the volume of input gas to the boiler burner proportionately to the changes in outside temperature.

Maximum performance and efficiency from such a boiler-water heating system is attained when the volume of fuel or gas input to the boiler burner is varied inversely proportional to the changes in outside temperature. That is, as the interior heating load requirement changes due to an increase or decrease in outside temperature, the volume of gas input to the boiler gas burner is respectively decreased or increased proportionately in a positive ratio to the outside temperature change. The system of the invention provides three stage control of the gas volume input to the burners at 50%, 75% and 100% of the required gas volume. Short cycling of the main gas burners during low demand periods is eliminated and instead the burners are operated at a reduced volume supply of fuel over a longer period of time resulting in enhancement of overall interior heating performance and balance of the heating medium, resulting in the ultimate in heating comfort.

It is therefore an object of the present invention to provide a construction of boiler fuel supply system in which the volume of input fuel is proportionately modulated by changes in outside temperature.

Another object of the present invention is to provide an automatically controlled boiler-water heating system which provides maximum interior comfort during periods of change in outside temperature.

Another object of the invention is to provide a heating control system in which boiler water temperature and boiler fuel input are modulated proportionately to the changes in outside temperature.

A further object of the invention is to provide a novel 3,399,832 Patented Sept. 3, 1968 heating system burner control system in which fuel is supplied to the same burner at three separate rates depending upon existing temperature conditions.

Still a further object of the invention is to provide a modified form of heating control system in which boilerwater temperature and boiler fuel input volume are modulated proportionately to outside temperature changes and changes in boiler-water temperature.

Other and further objects of the invention reside in certain novel circuit arrangements of the control system, and the operation of the boiler burner at gas volume inputs commensurate with surrounding conditions as set forth more fully in the specification hereinafter following by reference to the accompanying drawings, in which:

FIG. 1 is a schematic view showing the modulated gas volume, boiler water heating control system of the invention;

FIG. 2 is a simplified electrical schematic diagram of the system of FIG. 1; and

FIG. 3 is an electrical schematic diagram of a modilied form of the system of FIGS. 1 and 2.

Referring to FIGS. 1 and 2 of the drawings, the boiler water heating control system is powered by a transformer 1 having a primary winding connected across a source of AC. power 2, such as 120 volts 60 cycle line current, through a line disconnect switch 3. The secondary winding of transformer 1 has one side thereof commonly con nected to terminals 4, 5 and 6 of the solenoid windings of solenoid operated gas valves 7, 8 and 9, respectively. These three solenoid operated gas valves 7, 8 and 9 are respectively connected in gas lines 10, 11 and 12 connected between main gas line 13 and output manifold 14 connected at its terminal end to the main gas burners 15 of a water "boiler (not shown) of a hot water boiler heating system, or the like. Main gas line 13 is connected to a fuel or gas supply source 16 in the usual manner.

Solenoid gas valve 9 is connected in line 12 such that when the solenoid is deenergized there is no gas flame through line 12. However, when the solenoid gas valve is energized it opens gas line 12 such that the maximum 'volume of gas the system is designed to deliver at any given time flows from the main gas supply line 13, through line 12 and manifold 14 to gas burners 15. Gas line 12 and solenoid gas valve 9 thus supply 100% maximum gas flow rate to the burners.

Gas lines 10, 11 and 12 are preferably of the same diameter such that gas flame through all these pipes would normally be the same. However, a reduced gas flame orifice member 17 is connected in gas line 11 in series with solenoid gas valve 8, in the gas flow path through line 11. Gas flow orifice member 17 may be a separate member as indicated in FIG. 1 of the drawings or it may be incorporated directly in solenoid operated gas valve 8. Orifice member 17 comprises a restriction in gas line 11 which provides a positive reduced gas flow rate through line 11 to output manifold 14 and thus to the gas burners 1-5. The restriction in the line which may consist of a machine drilled orifice in member '17 is of such a dimension that when normally closed solenoid operated gas valve '8 is energized to open gas line 12, gas flows from main gas supply line 13, through line 11 and orifice member 17 to manifold 14 to deliver gas to main gas burners 15 at of the maximum gas flow rate. Thus, when gas valve 8 is operated, the heat supplied to the water boiler by gas burners 15 is not as intense as when gas valve 9 is operated.

In a similar manner, reduced gas flow orifice member 18 having a restriction therein of a size to deliver 50% of the maximum gas flow rate from main gas line '13 through gas line 10 and output manifold 14 to the main gas burners 15, is connected in gas line 10 in series with solenoid gas valve 7 in the gas flow path through line 10. When normally closed solenoid operated gas valve 7 is operated, line 10 is opened to supply gas to burners at a rate of 50% of the rate at which gas is supplied to burners 15 when solenoid operated gas valve 9 is operated.

Returning to the description of the electrical circuit, the opposite side of the secondary winding of transformer 1 is connected to one terminal of an indoor-outdoor reset control 19, which may be one of several such controls available on the commercial market. For purposes of illustration, control 19 may be reset control TA418A manufactured by Minneapolis-Honeywell Regulator Company, Minneapolis, Minn. This type of control is provided with an indoor (heating medium) temperature sensing bulb 20 mounted in the boiler water, in the usual manner, and an outdoor temperature sensing bulb 21 mounted outdoors in the usual manner, with the bulbs arranged to provide rapid response to boiler water and outdoor temperature changes. This control automatically operates to reset (raise or lower) the heating medium control point as outdoor temperatures vary below 70 F. Since controls of this type are well known in the art, the control has been shown schematically in the drawings as a switch connected in series circuit with one side of the secondary winding. The switch would be opened when the control point is satisfied and the proper balance between the heating medium temperature and the outdoor temperature is attained, otherwise the control maintains the circuit closed. Other safety circuit breaker type devices may be connected in series circuit with reset control 19, such as, high temperature limit boiler water detector control 22, low boiler water cut-otf control 23, and flame safeguard control 24. Each of these controls are well known in the art and are not essential to the operation of the control system of the invention, but may be connected in the circuit in the normal manner to cut off the heating system if certain potentially dangerous conditions are approached.

The control circuit also includes a pair of outdoor temperature controllers 25 and 26 having temperature sensing bulbs 27 and 28, respectively. Outdoor temperature controllers 25 and 26 are of the same type and may be any one of several such controllers well known in the are and commercially available. The controllers each have a single pole double throw switch, with the switch of outdoor temperature controller 25 including movable contact 29, normally closed fixed contact 30, and normally open fixed contact 31, and the switch of temperature controller 26 including movable contact 32, normally closed fixed contact 33, and normally open fixed contact 34. The movable contacts 29 and 32 are arranged to make or break a circuit on a rise or fall in temperature by the selection of the appropriate switch terminals. The movable contacts are operated by the temperature sensing bulbs 27 and 28, respectively, which by way of example may be liquid filled and connected to operate the movable contacts through a capillary system terminating in a bellows connected with the movable contact. The outdoor temperature controllers 25 and 26 may be adjusted to operate at individual selected temperatures. By way of example, temperature controllers 25 and 26 may be of the type manufactured by Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, under model N0. T675A.

The series circuit extending from indoor-outdoor reset control 19 is connected to movable contact 29 of outdoor temperature controller 25 and is also connected to terminal 36 of solenoid operated gas valve 9 through normally open disconnect switch 35. This switch is provided to by-pass outdoor temperature controllers 25 and 26 to energize solenoid operated gas valve 9 to set gas manifold pressure on starting procedure only. After the water boiler has been placed into operation, this switch is opened, as indicated in the drawings, and the three-stage fuel supply system of the invention is operational.

Normally closed fixed contact 30 of outdoor temperature controller 25 is connected to terminal 37 of the 50% solenoid operated gas valve 7, and normally open fixed contact 31 is connected to movable contact 32 of outdoor temperature controller 26. Normally closed fixed Contact 33 of this controller is connected to terminal 38 of solenoid operated gas valve 8, and the normally opened fixed contact 34 is connected to terminal 36 of rate solenoid operated gas valve 9.

Three controls are used to accomplish the desired modulating sequence, and the two dual-acting outdoor temperature controllers 2S and 26. The indoor-outdoor reset control 19 maintains and modulates the boiler water temperature in accordance with a change in outside temperature, that is, as the outside temperature varies, the temperature setting of control 19 automatically changes, thus raising or lowering the boiler water temperature. The dual-acting outdoor temperature controllers 25 and 26 vary the gas volume input to the boiler burners 15 by sensing through their temperature sensing bulbs 27 and 28, respectively, a change in outside temperature, and thus automatically open or close the three main solenoid operated gas valves 7, 8 and 9. As previously indicated, the first gas valve flow circuit incorporates an orifice member 18 which provides 50% of the maximum gas flow rate to burners 15. The second gas valve 8 flow circuit incorporates an orifice member 17 which provides 75% of the maximum 'gas flow rate to the burners 15, while the third gas valve 9 flow circuit delivers 100% of the maximum gas flow rate from the supply source to the burners 15.

In order to describe a typical sequence of operation of the system of the invention, assume the indoor-outdoor reset control 19 is set to 1-1 ratio, that is, as the temperature outside drops 1 F the set-point of control 19 is raised l F., or vice versa. Also assume the system is designed from 0 F. outside temperature to 70 F. inside temperature with a boiler water temperature of 180 F. at the 0 F. outside temperature. Outdoor temperature controller 25 is set to operate at 35 F., and outdoor temperature controller 26 is set to operate at 20 P. On a drop in temperature the switches of the controllers 25 and 26 actuate at the set-point minus the differential. The difierenti'al of the controllers is fixed at 1 F. With the system set up in this manner, the sequence of operation is as follows:

(1) When the outdoor temperature is 70 F. the boiler water temperature will be held at 180 F. minus 70 F. or F. At this point the transformer secondary circuit is completed through reset control 19, the other safety controls 22, 23 and 24 in the circuit, movable contact 29 and normally closed fixed contact 30 of outdoor temperature controller 25, and the coil of solenoid operated gas valve 7 represented by terminals 37 and 4, back to the other side of the secondary winding. Gas valve 7 is thus open in this condition and by reason of orifice member 18 allows 50% of the required gas volume to pass to burners 15 of the boilers. Reset control 19 combined with the minimum flow rate through gas valve 7 maintains the proper balance between boiler water temperature and outdoor temperature.

(2) The outside temperature now drops to 34 F. as the temperature drops, boiler water temperature gradually increases to 110 F. plus 35 F. (70 minus 35) or F. At 34" F. outside temperature, due to the connection between temperature sensing bulb 27 and movable contact 29, outdoor temperature controller 25 is actuated causing movable contact 29 to break electrical contact with normally closed fixed contact 30 and move into electrical contact with normally open fixed contact 31. This deenergizes 50% solenoid operated gas valve 7 causing the same to close and block gas flow through line 10, and actuate 75 solenoid operated gas valve 8 opening gas line 11 to increase the required volume of gas to burners 15 to 75% of the required gas volume.

One side of the secondary winding is connected to terminal 5 of gas valve 8 and the other side of the secondary winding is connected to terminal 38 of the gas valve through movable contacts 29 and fixed contact 31 of temperature controller 25, and movable contact 32 and normally closed fixed contact 33 of outdoor temperature controller 26, to energize gas valve 8. At this stage the reset control 19 together with the 75% gas flow rate to the burners maintains the proper balance between boiler water temperature and the outdoor temperature.

(3) As the outside temperature drops even further to 19 F., the indoor-outdoor reset control 19 gradually sets the boiler water temperature to 110 F. plus 50 F. (70 minus 20) or 160 F. At 19 F. outside temperature, since outdoor temperature controller 26 is set at 20 F., controller 26 is actuated by its temperature sensing bulb 28 moving movable contact 32 out of electrical contact with fixed contact 33 to deenergize 75% rate solenoid operated gas valve 8, into electrical contact with normally opened fixed contact 34 connected to terminal 36 of 100% rate solenoid operated gas valve 9 to energize this valve and open gas line 12 to increase the gas supply to boiler burners 15 through manifold 14 to the maximum or 100% rate. At this stage and at each stage of the operating sequence, the main boiler burners 15 will continue to operate until the set-point of reset control 19 is satisfied.

As the outdoor temperature increases, the sequence of operation of the control system is reversed to that set forth above such that the sequence of operation through the three gas supply stages causes a decrease of gas flow rate to the burners. The actuating points of the temperature controllers and 26 will be at the mentioned setpoints minus the differential.

A modified form of control system of the invention is shown in FIG. 3 wherein the electrical system for a proportional system is shown which is similar to the previous form of the invention described using outside air as the modulating medium, but in this form of the invention two modulating mediums are used in the system, that is, outside air and boiler water. The circuit of FIG. 3 may be superimposed on the diagram of FIG. 1 in the same manner as the circuit of FIG. 2, to provide a complete system diagram. It will be noted that the circuit of FIG. 3 is identical to the circuit of FIGS. 1 and 2 with the exception that in lieu of outdoor temperature controllers 25 and 2 6 a pair of dual-acting hot water controls 39 and 40 are utilized. Hot water controls 39 and 40 are of the type which include temperature sensing immersion bulbs 41 and 42, respectively, which are coupled to actuate single pole double throw switches within the controls. The arrangement of the movable and fixed switch contacts are the same as the movable and fixed contacts in controls 25 and 26 in FIG. 2 and the corresponding switch members have been indicated by corresponding primed numerals.

For maximum performance the gas volume input to the boiler gas burners 15 is varied directly to the change in boiler water temperature, that is, as the boiler water temperature increases or decreases due to heating load demand, the gas input to the boiler increases or decreases respectively.

As in the prior system, three controls are used in this system to accomplish the modulating sequence, the indoor outdoor reset control 19, the same as in the previous form of the system, and the two dual-acting hot water controls 39 and 40. These controls are arranged to make or break contacts on a rise in water temperature and several such controls are commercially available. One such hot water control which is operable in the circuit of the invention is Model No. L6006A, manufactured by Minneapolis-Honeywell Regulator Company, Minneapolis, Minn. The indoor-outdoor reset control 19 maintains and modulates the boiler water temperature in accordance with a change in outside temperature, that is, as the outside temperature varies, the temperature setting of control 19 automatically changes, thus raising or lowering boiler water temperature. The change in boiler water temperature is then sensed by the dual-acting hot water controls 39 and 40, through their respective immersion bulbs 41 and 42, and the controls react at predetermined or preselected temperature settings to vary the gas rate input to the boiler burners 15 by opening or closing the three automatic operating solenoid operated gas valves 7, 8 and 9. As in the previous form of the invention, the first gas valve 7 flow circuit incorporates an orifice member 18 (FIG. 1) to provide 50% of the maximum gas flow rate to the burners. The second gas valve 8 flow circuit incorporates an orifice member 17 such that when the valve is energized it provides of the maximum gas flow rate to the burners, while the third gas valve 9 in its energized state delivers or the maximum gas flow rate to the gas burners 15.

For purposes of explaining the sequence of operation of this system, assume, as in the first system, the indooroutdoor reset control 19 is set to a l-l ratio, that is, as the temperature outside drops 1 F., the set point of the reset control 19 is raised 1 F., or vice versa as the outside temperature rises. Also assume that the system is designed 'from 0 F. outside temperature to 70 F. inside temperature with a boiler water temperature of 180 F. at the 0 F. outside temperature condition. Hot water control 39 is set to operate at a temperature setting of 145 F., while hot water control 40 is set to operate at a temperature setting of 160 F., with these controls being actuated by the bulbs 41 and 42, respectively, immersed in the water of the boiler (not shown).

With these assumed conditions, the sequence of system operation is as follows:

(1) When the outdoor temperature is 70 F., the boiler water temperature will be held at 180 F.-70 F. or F. by means of reset control 19 which maintains the proper balance between boiler water temperature and outdoor temperature. At this stage solenoid gas valve 7 is energized through reset control 19 and contacts 29', 30' of hot water control 39 to allow the boiler burners 15 to ope-rate on 50% of the maximum gas flow rate. The main burners 15 will continue to operate in this condition until the reset control 19 set-point is satisfied.

(2) As the outdoor temperature drops to 35 F., the boiler water temperature gradually increases to 110 F. plus 35 F. (70 F.-35 F.) or F. which is the temperature setting of the operating point of hot water control 39. When temperature sensing bulb 41 senses this water temperature, it actuates control 39 and movable contact 29' breaks contact with fixed contact 30' deenergizing the 50% rate gas valve 7, and makes contact with normally open fixed contact 31' which energizes the 75% rate solenoid ope-rated gas valve 8 through contacts 32, 33' of hot water control 40. The gas supply rate to burners 15 of the water boiler is thus increased and the burners will continue to operate until the set-point of indoor-outdoor reset control 19 is satisfied.

(3) As outdoor temperature drops still "further to 20 F., the indoor-outdoor reset control 19 automatically resets its set-point to 110 F., plus 50 F. (70 F.-20 F.) or F. to raise the boiler water to this temperature, which is the temperature setting operating point of hot water control 40'. When the boiler water temperature reaches this point as sensed by temperature sensing immersion bulb 42, hot water control 40 is operated opening contacts 32', 33, to deenergize and close 75 rate solenoid operated gas valve 8, and closing contacts 32', 34' to energize and open 100% rate solenoid operated gas valve 9 to increase the required volume of gas flow to the burners 15 to a maximum. The boiler burners will continue to operate in this condition until the setpoint of reset control 19 is satisfied.

As the outdoor temperature rises, the sequence of system operation is reversed to that set forth above and it is believed unnecessary to trace the reverse operating sequence.

While the invention has been disclosed and described in certain preferred embodiments, it is realized that modifications can be made without departing from the spirit of the invention and it is to be understood that no limitations upon the invention are intended other than those imposed by the scope of the appended claims.

What is claimed is:

1. In a control system for :a heating system with fluid fuel burning means, the combination of; a main fuel supply line; an output supply line connected to deliver fuel to said fluid 'fuel burning means; first, second, and third electrically operated valve means individually connected to supply fuel from said main fuel supply line to said output supply line when energized; said second and third electrically operated valve means each connected to deliver fuel to said fuel burning means through said output supply line at individually dilferent rates than said first electrically operated valve means; power supply means commonly connected with said first, second and third electrically .operated valve means; indoor-outdoor reset thermostat control means, having a temperature control setting which is automatically variable with changes in outdoor temperatures about a preselected boiler water temperature setting, and having switch means operable to open and closed positions by said control setting; first temperature controlled switching means operable at a first predetermined temperature assuming a first position at temperatures above said first predetermined temperature and a second position at temperatures below said first predetermined temperature; said switch means of said indoor-outdoor reset thermostat control means and said first temperature controlled switching means connected in series circuit with said power supply means; said third electrically operated valve means connected in circuit with said first position of said first temperature controlled switching means to be energized to deliver fuel at one rate to said burning means at temperatures above said first predetermined temperature; second temperature controlled switching means connected in circuit with said second position of said first temperature controlled switching means, and operable at a second predetermined temperature assuming a first position at temperatures above said second predetermined temperature and a second position at temperatures below said second predetermined temperature; said second electrically operated valve means connected in circuit with said first position of said second temperature controlled switching means to be energized to deliver fuel at another rate to said burning means at temperatures above said second predetermined temperature; and said third electrically operated valve means connected in circuit with said second position of said second temperature controlled switching means and operable to deliver fuel at still another rate to said burning means at temperatures below said second predetermined temperature; whereby fuel is delivered at three different rates to said burning means under varying temperature conditions.

2. A control system as set 'forth in claim 1 in which said indoor-outdoor reset thermostat control means includes a boiler water temperature sensor and an outdoor temperature sensor.

3. A cont-r01 system as set forth in claim 1 in which said second temperature controlled switching means is connected to operate at a temperature below the operating temperature of said first temperature controlled switching means.

4. A control system as set forth in claim 3 in which said first electrically operated valve means is constructed to deliver fuel at a maximum rate to said fuel burning means, said third electrically operated valve means is constructed to deliver fuel to said fuel burning means at a rate substantially half that of said first electrically operated valve means, and said second electrically operated valve means is constructed to deliver fuel to said fuel burning means at a rate intermediate the rates of said first and third valve means.

5. A control system as set forth in claim 1 in which said first, second and third electrically operated valve means are connected to said first and second temperature controlled switching means for energization by said power supply means one at a time.

6. A control system as set forth in claim -1 in which said first and second temperature controlled switching means each includes an outdoor temperature sensing means connected to operate said respective temperature controlled switching means, upon change in outdoor temperature beyond the first and second predetermined temperature settings.

7. A control system as set forth in claim 1 including a water boiler heated by said fuel burning means; individual sensing means, connected in said water boiler in communication with the boiler water therein to sense the temperature thereof, connected to respectively control the operation of said first and second temperature controlled switching means as the boiler water temperature exceeds said first and second predetermined temperatures; whereby said electrically operated valve means are modulated by outdoor temperature and boiler water temperature.

8. A control system as set forth in claim 1 in which said first temperature controlled switching means is in the second position thereof when said second temperature controlled switching means is in said respective second position.

9. A control system as set forth in claim 1 in which said first temperature controlled switching means is set to operate to the respective second position at a temperature higher than said second temperature controlled switching means is set to operate to the respective second position.

10. A control system as set forth in claim 1 in which said second electrically operated valve means is operated to supply fuel to said output supply line at an increased rate from said third electrically operated valve means in a temperature range between said first and second predetermined temperatures, when said first temperature controlled switching means is in said second position and said second temperature controlled switching means is in the respective first position, until the variable setting of said indoor-outdoor reset thermostat control means is satisfied.

References Cited UNITED STATES PATENTS EDWARD J. MICHAEL, Primary Examiner. 

1. IN A CONTROL SYSTEM FOR A HEATING SYSTEM WITH FLUID FUEL BURNING MEANS, THE COMBINATION OF: A MAIN FUEL SUPPLY LINE; AN OUTPUT SUPPLY LINE CONNECTED TO DELIVER FUEL TO SAID FLUID FUEL BURNING MEANS; FIRST, SECOND, AND THIRD ELECTRICALLY OPERATED VALVE MEANS INDIVIDUALLY CONNECTED TO SUPPLY FLUEL FROM SAID MAIN FUEL SUPPLY LINE TO SAID OUTPUT SUPPLY LINE WHEN ENERGIZED; SAID SECOND AND THIRD ELECTRICALLY OPERATED VALVE MEANS EACH CONNECTED TO DELIVER FUEL TO SAID FUEL BURNING MEANS THROUGH SAID OUTPUT SUPPLY LINE AT INDIVIDUALLY DIFFERENT RATES THAN SAID FIRST ELECTRICALLY OPERATED VALVE MEANS; POWER SUPPLY MEANS COMMONLY CONNECTED WITH SAID FIRST, SECOND AND THIRD ELECTRICALLY OPERATED VALVE MEANS; INDOOR-OUTDOOR RESET THERMOSTAT CONTROL MEANS; HAVING A TEMPERATURE CONTROL SETTING WHICH IS AUTOMATICALLY VARIABLE WITH CHANGES IN OUTDOOR TEMPERATURES ABOUT A PRESELECTED BOILER WATER TEMPERATURE SETTING, AND HAVING SWITCH MEANS OPERABLE TO OPEN AND CLOSED POSITIONS BY SAID CONTROL SETTING; FIRST TEMPERATURE CONTROLLED SWITCHING MEANS OPERABLE AT A FIRST PREDETERMINED TEMPERATURE ASSUMING A FIRST POSITION AT TEMPERATURES ABOVE SAID FIRST PREDETERMINED TEMPERATURE AND A SECOND POSITION AT TEMPERATURES BELOW SAID FIRST PREDETERMINED TEMPERATURE; SAID SWITCH MEANS OF SAID INDOOR-OUTDOOR RESET THERMOSTAT CONTROL MEANS AND SAID FIRST TEMPERATURE CONTROLLED SWITCHING MEANS CONNECTED IN SERIES CIRCUIT WITH SAID POWER SUPPLY MEANS; SAID THIRD ELECTRICALLY OPERATED VALVE MEANS CONNECTED IN CIRCUIT WITH SAID FIRST POSITION OF SAID FIRST TEMPERATURE CONTROLLED SWITCHING MEANS TO BE ENERGIZED TO DELIVER FUEL AT ONE RATE TO SAID BURNING MEANS AT TEMPERATURES ABOVE SAID FIRST PREDETERMINED TEMPERATURE; SECOND TEMPERATURE CONTROLLED SWITCHING MEANS CONNECTED IN CIRCUIT WITH SAID SECOND POSITION OF SAID FIRST TEMPERATURE CONTROLLED SWITCHING MEANS, AND OPERABLE AT A SECOND PREDETERMINED TEMPERATURE ASSUMING A FIRST POSITION AT TEMPERATURES ABOVE SAID SECOND PREDETERMINED TEMPERATURE AND A SECOND POSITION AT TEMPERATURES BELOW SAID SECOND PREDETERMINED TEMPERATURE; SAID SECOND ELECTRICALLY OPERATED VALVE MEANS CONNECTED IN CIRCUIT WITH SAID FIRST POSITION OF SAID SECOND TEMPERATURE CONTROLLED SWITCHING MEANS TO BE ENERGIZED TO DELIVER FUEL AT ANOTHER RATE TO SAID BURNING MEANS AT TEMPERATURES ABOVE SAID SECOND PREDETERMINED TEMPERATURE; AND SAID THIRD ELECTRICALLY OPERATED VALVE MEANS CONNECTED IN CIRCUIT WITH SAID SECOND POSITION OF SAID SECOND TEMPERATURE CONTROLLED SWITCHING MEANS AND OPERABLE TO DELIVER FUEL AT STILL ANOTHER RATE TO SAID BURNING MEANS AT TEMPERATURES BELOW SAID SECOND PREDETERMINED TEMPERATURE; WHEREBY FUEL IS DELIVERED AT THREE DIFFERENT RATES TO SAID BURNING MEANS UNDER VARYING TEMPERATURE CONDITIONS. 